In active power switching applications, a decision/controller circuit is provided to control two switches to select between one of two power supplies having a higher voltage. These decision/controller circuits typically are connected to a power terminal on the output of the two switches to receive the operating power therefrom. In operation, these decision/controller circuits receive as comparison inputs the voltages output from each of the power supplies. Two control outputs are added to control each of the two switches, depending upon the relative input level.
When the two power supplies comprise batteries, the decision/controller circuit must make a determination as to which of the power supplies is the highest, this being some indication that the battery with the highest charge is given preference over the other. However, after charge has been depleted from a battery, the voltage thereof will be reduced and then the decision/controller circuit is operable to switch to the other battery. When the charge in this battery has been depleted below that of the first battery, the decision/controller circuit will then switch back to the first battery. This will continue until the charge in both batteries have been depleted, thus providing some equalization in the charge depletion of both batteries. One system that discloses such a device is illustrated in U.S. Pat. No. 4,645,943.
One disadvantage to the prior art systems for determining the switching point of the decision/controller circuit between the two batteries is the current requirements therefore. These circuits specifically utilize some type of diode configured devices that drive a load resistor. When the voltage is near a switching threshold, the diode typically is highly conductive just prior to switching. However, when the battery is connected, this typically indicates a battery backed-up condition which can continue for a long duration of time. As such, the decision/controller circuit can be in a high battery drain condition for a long duration of time, proximate to the point where the system switches from one battery to another. As a result, these prior systems have significant disadvantages with respect to battery drain or current limiting resistor value/area during the operation thereof.